The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for optimizing a substrate in a plasma processing system.
In the processing of a substrate, e.g., a semiconductor wafer or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate (chemical vapor deposition, plasma enhanced chemical vapor deposition, physical vapor deposition, etc.) for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck.
Appropriate plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6, etc.) are then flowed into the chamber and ionized by a first RF energy source, commonly coupled to a RF coupling structure, such as a set of induction coils. Additionally, a second RF energy source may also be coupled to the substrate in order to create a bias with the plasma, and direct the plasma away from structures within the plasma processing system and toward the substrate.
The induction coil is a device, similar in purpose to a transformer, that induces a time-varying voltage and potential difference in the plasma processing gases to create a plasma by successively turning the current on and off in the primary coil. A common configuration is to place a planar coil at the top of the chamber, also called the TCP™ (e.g., transformer coupled plasma). Another configuration is to configure the plasma processing system so that a solenoidal coil is wound around the side of the plasma processing chamber.
Plasma is generally comprised of partially ionized gas. Because the plasma discharge is RF driven and weakly ionized, electrons in the plasma are not in thermal equilibrium with ions. That is, while the heavier ions efficiently exchange energy by collisions with the background gas (e.g., argon, etc.), electrons absorb the thermal energy. Because electrons have substantially less mass than that of ions, electron thermal velocity is much greater than the ion thermal velocity. This tends to cause the faster moving electrons to be lost to surfaces within the plasma processing system, subsequently creating positively charged ion sheath between the plasma and the surface. Ions that enter the sheath are then accelerated into the surface.
Lower RF frequencies tend to cause plasma ions to cross the sheath in less than one RF cycle, creating large variations in ion energy. Likewise, higher RF frequencies tend to cause plasma ions take several RF cycles to cross the sheath, creating a more consistent set of ion energies. Higher frequency tends to result in lower sheath voltages than when excited by a lower frequency signal at a similar power level.
Coupled between the RF source and the plasma processing chamber is commonly a matching network. Generally, a matching network transforms the complex impedance of the plasma, as viewed from the transmission line termination, to the nominal output impedence of the RF generator. For example, if an RF generator is delivering an output power of 2 kW (called incident or forward power) and the matching network is not properly “tuned” (resulting, for instance, in 50% reflected power), then 1 kW of power will be reflected back to the RF generator. This means that only 1 kW is delivered to the load (plasma chamber). The combination of a high quality, low impedance, properly selected length transmission line with a properly sized (for current and impedance range) matching network can provide the best power transfer from generator to the plasma chamber.
Referring now to FIG. 1, a simplified diagram of a plasma processing system 100 is shown. Generally, an appropriate set of gases is flowed into chamber 102 through an inlet 108 from gas distribution system 122. These plasma processing gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor wafer or a glass pane, positioned on an electrostatic chuck 116. Upper chamber plate 120, along with liner 112, helps to optimally focus plasma 110 onto substrate 114.
Gas distribution system 122 is commonly comprised of compressed gas cylinders 124a–f containing plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4HBr, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6, etc.). Gas cylinders 124a–f may be further protected by an enclosure 128 that provides local exhaust ventilation. Mass flow controllers 126a–f are commonly a self-contained devices (consisting of a transducer, control valve, and control and signal-processing electronics) commonly used in the semiconductor industry to measure and regulate the mass flow of gas to the plasma processing system.
Induction coil 131 is separated from the plasma by a dielectric window 104, and generally induces a time-varying electric current in the plasma processing gases to create plasma 110. The window both protects induction coil from plasma 110, and allows the generated RF field to penetrate into the plasma processing chamber. Further coupled to induction coil 131 at leads 130a–b is matching network 132 that may be further coupled to RF generator 138. As previously described, matching network 132 attempts to match the impedance of RF generator 138, which typically operates at 13.56 MHz and 50 ohms, to that of the plasma 110.
Referring now to FIG. 2, a simplified diagram of a TCP™ induction coil is shown. An induction coil may be fabricated from high conductivity copper tubing—generally circular, rectangular, or square, dependant upon the application, and must be rugged to withstand constant usage. As shown in FIG. 1, leads 130a–b are used to couple induction coil 131 to matching network 132.
As the chamber pressure or power levels change, however, the matching network and load may together become unstable. The result is a rapid fluctuation or jittering that changes faster than the matching network can respond. The resulting power transfer instability can both damage components of the matching network and the RF generator, and substantially affect the production yield of the substrate.
In view of the foregoing, there are desired methods and apparatus for optimizing a substrate in a plasma processing system.